The invention relates to a device and a method for cooling a platform of a turbine blade comprising a blade root, a blade surface with a leading and trailing edge, as well as a blade tip with a platform, through which platform extends radially, at least in part, at least one cooling channel that is connected with at least one outlet channel exiting via an outlet opening at the platform.
Cooling problems of the previously mentioned type occur in particular in turbine blades used in gas turbine systems. In particular, in the individual gas turbine stages, the hot gases generated inside the combustor flow around the turbine blades. In order to prevent overheating of turbine blades in operation, the aspect of targeted cooling of gas turbine blades plays an important role in the design and construction of such systems. Usually, part of the air precompressed in the compressor stage is removed in a targeted manner for cooling purposes and is therefore removed from the further combustion process. Rather, the cooling air reaches the area of the turbine stages via cooling channel systems provided both in rotating as well as stationary system components in order to cool the system components directly exposed to the hot gases. In order to cool the rotating blades arranged in a plurality of rotating blade rows positioned axially behind each other, the rotating blades have radial cooling channels through which cooling air fed in from the rotor arrangement is guided longitudinally to the turbine blade surfaces, exits through cooling air openings provided accordingly on the rotating blade surface, and mixes with the hot gases.
In some cases, turbine blades have platforms or so-called shrouds on their radial side facing away from the rotor arrangement in order to minimize leakage flows that are able to form between the turbine blade tips and the stationary system components. In the same way, such platforms and shrouds help in effectively dampening vibrations that form along the turbine blades during the operation of the gas turbine.
For the cooling of such platforms, U.S. Pat. No. 5,482,435 describes a cooling channel system within a platform, through which cooling air is guided and in this way effectively helps to cool the platform. The cooling air passes through a central cooling channel oriented radially towards the turbine blade into the area of the platform where said cooling air is discharged to the outside via two partial channels. The partial cooling channels provided in the platform extend in such a way that the cooling air exiting from the platform is oriented almost vertically to the main flow direction of the hot gases flowing through the gas turbine. On the one hand, this has the result, however, that the flow behavior of the main flow is significantly irritated, so that the aerodynamic efficiency is reduced. On the other hand, the cooling air exiting from the platform is unable to contribute to any energy yielding or improved energy conversion inside the gas turbine.
The invention is based on the objective of further developing a device as well as a method for cooling a platform of a turbine blade in such a way that the main flow acting directly on the turbine blade is impaired as little as possible in order not to aggravate the aerodynamic conditions within the turbo-machine. Rather, the goal is to achieve, in addition to the previously mentioned effective cooling effect, an additional energy yield by means of the exit of the cooling air from the platform.
According to the invention, a device for cooling a platform of a turbine blade comprising a blade root, a blade surface with a leading and trailing edge, as well as a blade tip with a platform, through which platform extends radially, at least in part, at least one cooling channel that is connected with at least one outlet channel exiting via an outlet opening at the platform, is further developed in such a way that the outlet channel has, adjacent to the outlet opening, a longitudinal channel direction that extends, in projection, longitudinally to the turbine blade in an essentially co-parallel manner with respect to the flow direction of a local flow field of a mass flux relatively passing by the turbine blade, said flow field directly flowing over the exit opening.
In principle, the cooling device according to the invention can be used for all turbine blades provided with a platform. The advantages connected with the measure according to the invention are explained in more detail below in reference to the example of the turbine guide blade inside a gas turbine system. Naturally, it would also be possible to use the cooling device according to the invention with platforms of stationary guide blades. The measure according to the invention is not restricted to the use of turbine blades inside gas turbine stages of gas turbine systems, but can be used in all turbo-machines in which similar cooling problems occur, for example, inside compressors or similar turbo-machines.
The arrangement of the exit channel according to the invention inside the platform, through which the cooling air exits through an exit opening, is, according to the invention, oriented in such a way that the cooling air flowing from the platform preferably has the same flow direction with which the main flow of the hot gases flows around the turbine blade and therefore around the platform itself. If the exit opening of the outlet channel is provided on the platform top side radially facing away from the turbine blade surface, the cooling channel preferably extends at a slight angle in relation to the platform top side. Alternatively, the exit opening may be positioned on the closing edge of the platform facing away from the flow, so that the cooling air flowing out of the platform is oriented co-parallel to the hot gases flowing around the platform. The exit opening of the cooling channel is located on the platform preferably downstream in relation to the leading edge of the turbine blade so that it is ensured that a cooling channel section as long as possible extends inside the platform so that the most effective cooling effect can be achieved.
Cooling measures inside the platform, which platform, in the case of rotating turbine blades, is subject to high centrifugal forces because of its radial spacing with respect to the rotation axis, make an important contribution to positively influencing the creeping behavior of the blade material in the area of the platform, i.e., any buckling and deformation of material as a result of a softening of the material with simultaneous action of high centrifugal forces is reduced or eliminated with effective cooling measures. With the help of the cooling measure according to the invention inside the platform, a creeping of the material can be significantly reduced.
The main advantage associated with the cooling channel system inside the platform is, however, the additional energy yield that can be achieved with the targeted, co-parallel flow exit of the cooling air relative to the main flow that flows around the turbine blade. It was found, for example, that the cooling air flowing out of the cooling channel oriented according to the invention flows through the exit opening on the platform, contributes to a measurable energy yield that is the result of the cooperation of an additional impulse contribution for driving the turbine blade and a relatively negligible irritation or impairment of the main flow of the hot gases flowing around the turbine blade.
It is preferred that a plurality of correspondingly oriented cooling channels be positioned inside a platform, so that the previously described, advantageous effects with respect to cooling effect and additional energy contribution can be increased. Additional details with respect to possible exemplary embodiments can be found in detail in the following exemplary embodiments.
To produce the platform constructed according to the invention, a number of known techniques can be used to produce the cooling channel or a plurality of correspondingly oriented cooling channels into the platform. Especially suitable for this purpose are EDM processes (electro-discharge machining) and also conventional drilling techniques using laser beams, electrochemical processes, as well as water jet techniques.
Naturally, it is also possible to provide platforms of turbine blades at their respective turbine blade roots with correspondingly oriented cooling channels. Although the aspect of an additional energy yield plays only a minor role for platforms in the blade root area, the exiting cooling air, as a result of the corresponding exit openings, does not or does only insignificantly impair the main flow, even in the area of the blade roots.